In a satellite television communication system, the satellite receives a signal representing audio, video, or data information from an earth-based transmitter. The satellite amplifies and rebroadcasts this signal to a plurality of receivers, located at the homes of consumers, via transponders operating at specified frequencies and having given bandwidths. Such a system includes an uplink transmitting portion (earth to satellite), an earth-orbiting satellite receiving and transmitting unit, and a downlink portion (satellite to earth) including a receiver located at the user's residence. The subject matter of the present invention is especially concerned with a downlink receiving unit designed for relatively easy installation at the user's residence.
The present system utilizes two satellites within a few degrees of each other in geosynchronous earth-orbit stationed at an altitude of 22,300 miles, approximately over the state of Texas. With this arrangement, receivers located anywhere in the contiguous 48 states of the United States can receive signals from both satellites on the same receiving antenna dish without having to reposition the antenna dish. Each satellite transmits its signals with a respective polarization. Selecting a satellite for reception of its signals is accomplished at the receiving antenna by selecting those signals with the appropriate polarization. Each satellite includes sixteen transponders for transmitting signals to the receiving antenna dish over a range of frequencies. Each transponder is time-multiplexed to convey a plurality of television channels (e.g., six to eight channels), substantially simultaneously. The satellite signals are transmitted in compressed and packetized form. and comprise television and ancillary data signals. Because the system is capable of carrying as many as two hundred fifty-six channels, some television program selection method and apparatus, which is easy to understand and operate, must be provided for the user.
If we look to conventional analog VHF and UHF broadcast television as a guide, we find that the solution provided therein is of little help, for the following reasons. The channel number of a given television station corresponds to a fixed band of frequencies. In other words, channel 6 in the United States is regulated to occupy the range from 82-88 MHz. Most non-technical consumers have no understanding of the frequency allocations of the television broadcast bands. Instead, they tune a desired channel by entering its channel number into their receiver. Their receiver is programmed with the proper information to perform the required tuning to the desired channel by generating the appropriate bandswitching and tuning commands, in response to the entering of the channel number by the user. It is possible for manufacturers to build a fixed channel number-to-frequency translation arrangement into each television receiver, only because the relationship between channel number and frequency band must conform to a broadcast standard.
This fixed-frequency standard is acceptable to the broadcasters because their transmitting equipment is readily accessible for maintenance purposes due to its location on the ground. If the transmitter malfunctions, it can be repaired and the station can be back "on-the-air" at its designated frequency band in a relatively short time. In contrast, a fixed-frequency arrangement for a satellite is undesirable because of the practical inaccessibility of an orbiting satellite. In the event that a transponder malfunctions, that transponder is thereafter inoperative, essentially forever, and receivers programmed to tune that transponder to receive a desired television program would not receive a usable signal. In such an event, the receiver will have lost the desired television channels and will have no idea how to find them.
A satellite receiver may be programmed to perform a function similar to the common autoprogramming function, in which a television receiver searches for all active channels and records detection of each as it is found. If such a system is used after a transponder failure, the failed transponder will be noted and a new active transponder will be found (assuming that the programming has been moved to a new transponder by ground-based control personnel). The user's receiver would then have to perform an internal remapping to associate the desired channel with the new transponder. However, in the event that a power supply module failed in the satellite, several transponders which may receive power from that module may cease transmitting at once. In such an event, the autoprogramming solution given above will not work because several new transponders will be found at the same time as several old transponders are noted as missing. In such a case, the receiver will have no way of allocating the received signals to their proper channels. Moreover, as noted above, since each transponder conveys six to eight channels, the channels assigned to the failed transponder may be distributed among several still-functioning transponders. In that case the receiving antenna will have access to all of the television channels, but the receiver will, quite literally, not know where to look for those channels which have been moved.